Absorption of 10 Gev--1 Tev Gamma Rays by Radiation from Broad-Line Region in 3C 279

Abstract

In this paper, we study the photon-photon pair production optical depth for 10 GeV--1 TeV gamma rays from 3C 279 due to the diffuse radiation of broad-line region (BLR). Assuming a power-law spectrum of $E_{\gamma }^{-a_2}$ for the photon intensity of very high energy (VHE) gamma rays, $a_1\gtrsim 405$ and $a_2\gtrsim 6.4$ are inferred by the integrated photon fluxes measured by MAGIC and HESS. Based on this power-law spectrum, the pre-absorbed spectra are inferred by correcting the photon-photon absorption on the diffuse photons of the BLR (internal absorption) and the extragalactic background light (external absorption). Position of gamma-ray emitting region $R_{\gamma }$ determines the relative contributions of this two diffuse radiation to the total absorption for 10 GeV--1 TeV gamma rays. The internal absorption could make spectral shape of gamma rays more complex than only corrected for the external absorption, and could lead to the formation of arbitrary softening and hardening gamma-ray spectra. It should be necessary for the internal absorption to be considered in studying 10 GeV--1 TeV gamma rays from powerful blazars. The energies of annihilated gamma-ray photons due to the internal absorption are likely to be mainly reradiated around GeV. Our results indicate that $R_{\gamma }$ may be between the inner and outer radii of the BLR for 3C 279. This implies for powerful blazars that $R_{\gamma }$ might be neither inside the BLR cavity nor outside the BLR, but be within the BLR shell. Observations of $GLAST$, MAGIC, HESS, and VERITAS in the near future could give more constraints on the position of the gamma-ray emitting region relative to the BLR.

arXiv comment29 pages, 7 figures, Received: 4 June 2008, Accepted in ApJ: 18 Jul 2008

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